1. Field of the Invention
The present invention is related to an antenna, for instance, an indoor-purpose antenna and an outdoor-purpose antenna, which are used so as to receive and communicate terrestrial integrated services digital broadcasting waves in the UHF frequency band.
2. Background
In the ISTB-T (Terrestrial Integrated Services Digital Broadcasting) system, electromagnetic waves of the UHF frequency band are utilized, and in this UHF frequency band, 470 to 770 MHz (13 to 62 channels) are used.
As indoor-purpose antennas which receive electromagnetic waves of the UHF frequency band, loop antennas and dipole antennas have been employed (refer to, for example, JP-A-7-249922). The dipole antennas are constituted by conductive pipes, while broadband characteristics and high gain characteristics of the dipole antennas are known in the field. However, entire lengths of these dipole antennas require approximately 0.5λ (wavelength) of lower end frequencies, and radiation characteristics thereof are a single directivity characteristic.
Also, as outdoor-purpose antennas which receive electromagnetic waves of the UHF band, single directivity antennas have been used which are typically known as Yagi type antennas and reflector-equipped dipole antennas, while these single directivity antennas represent superior reception performance with respect to receptions of a specific direction. However, since the outdoor-purpose antennas require large occupied areas and also have the single directivity characteristic, in such a case that directions of traveling electromagnetic waves are different from each other depending upon broadcasting stations, the outdoor-purpose antennas are required to be separately installed toward the respective directions of the traveling electromagnetic waves.
FIG. 14 represents an example of such a case that two pieces of Yagi-type antennas have been installed in correspondence with electromagnetic waves whose traveling directions are different from each other.
The Yagi-type antennas 1a and 1b for horizontally polarized waves are mounted on a summit portion of an antenna mast 2 and are separated in a predetermined interval. In this case, two pieces of the Yagi-type antennas 1a and 1b are set to be directed toward the traveling directions of the electromagnetic waves. Power feeding cables 4a and 4b are connected to feeding points 3a and 3b of the Yagi-type antennas 1a and 1b respectively. The power feeding cables 4a and 4b are held along the antenna mast 2, and are connected to a mixer 5 which is mounted on a half way of this antenna mast 2. In this mixer 5, signals received by the Yagi-type antennas 1a and 1b are mixed with each other, and then, the mixed signal is supplied to a TV receiver set in a home through an output cable 6. It should be understood that a holding member 7 is mounted on a base of the antenna mast 2, while the holding member 7 is employed so as to fix this antenna mast 2 on, for example, a roof.
As previously described, in such a case that electric magnetic waves whose traveling directions are different from each other depending upon broadcasting stations are received by employing single directivity antennas, a plurality of such single directivity antennas must be installed, and thus, antenna constructions are very cumbersome, for instance, mixers are installed, and cable wirings become complex.
Also, in the above-described UHF-band broadband antennas, since upper projection areas are large, snow may be easily accumulated thereon, and thus, strengths of the antennas themselves must be increased in order to endure electric influences caused by the accumulations of snow and weights given by the accumulated snow.
Also, since the above-described Yagi-type antennas have the single directivity characteristics, in such a case that traveling directions of electromagnetic waves are different from each other depending upon broadcasting stations, the plurality of these antennas are required to be separately installed toward the respective traveling directions of the electromagnetic waves. Accordingly, there is such a problem that installation places are limited, and further, installation costs are increased.
Also, in order to simply install antennas, indoor-purpose antennas have been commercially provided in markets. Similarly, since these indoor-purpose antennas have directivity, the antenna main bodies must be rotated and be adjusted in order to achieve better reception conditions thereof. A lengthy time is required to seek best receiving conditions. Then, in such a case that indoor-purpose antennas are installed beside television receivers, best reception directions of these indoor purpose antennas are not always made coincident with the directions of the television receivers, which may largely damage good appearances. Moreover, antennas having single directivity characteristics have such a problem that if a subject having a certain dielectric constant, for instance, a person approaches the antennas, then reception levels thereof are largely lowered.
An object of the present invention is to provide an indoor and outdoor commonly-used antenna which can be made compact with a simple structure, can be easily installed even in a narrow installation space, and can be operated by a single piece of antenna even in such a case that electromagnetic waves are traveled from a plurality of directions, and further, which can be utilized for an indoor-purpose antenna as well as an outdoor-purpose antenna.